Refining the Substrate-Cofactor Disposition Model of Hyoscyamine 6β-Hydroxylase Catalysis Using Hyoscyamine Analogs

Area of Science:

• Biochemistry
• Enzymology
• Organic Chemistry

Background:

• Hyoscyamine 6β-hydroxylase (H6H) is a key enzyme in tropane alkaloid biosynthesis, specifically scopolamine production.
• H6H is a mononuclear nonheme iron and 2-oxoglutarate-dependent oxidase with dual catalytic functions: C6-hydroxylation and epoxidation.
• Existing models propose substrate disposition dictates reaction outcomes based on intermediary iron complexes.

Purpose of the Study:

• To investigate the role of substrate structure in H6H's dual catalytic functions.
• To test the accuracy of the substrate disposition model in predicting reaction outcomes.
• To identify additional factors influencing H6H's enzymatic activity.

Main Methods:

• Synthesis and assay of various H6H substrate analogs with modifications at C6 and C7 positions.
• Enzymatic assays using H6H to evaluate catalytic activity and product formation.
• Analysis of reaction products to determine hydroxylation and epoxidation efficiency.

Main Results:

• Substrate analogs with cyclopropyl, methylidene, fluoro, methoxy, and trifluoromethoxy groups were synthesized and tested.
• Results indicate that the substrate disposition model alone does not fully predict reaction outcomes.
• Evidence suggests H-bonding interactions stabilizing intermediate complexes play a crucial role.

Conclusions:

• H6H's reaction outcomes are influenced by factors beyond simple substrate disposition.
• Hydrogen bonding interactions are critical for stabilizing intermediate hydroxy-ferric complexes.
• These findings refine our understanding of nonheme iron enzyme mechanisms and substrate-cofactor interactions.